Optional-dependency docs: add a self-dependency example

[AlexWaygood]

Hi, thanks for hatch/hatchling!

This PR adds a short example to the docs for how to add a self-dependency when dealing with optional feature groups. It took me a while to figure out how to do this, and this example would have helped me.

From the sound of it, it seems like this is an officially supported feature of Python packaging and won't be going away anytime soon: https://discuss.python.org/t/pyproject-toml-optional-dependencies-redundancy-aka-dry-extras/8428/6

[AlexWaygood]

I don't think any of the ruff complaints are caused by this PR -- they all seem to be linting errors in .py files, and this PR only touches the docs.

[Bobronium]

This doesn't work for me when my project name overlaps with other project available on PyPI (though I'm installing it in editable mode):

[project]
name = "chatbot"

[project.optional-dependencies]
profiling = ["pyinstrument"]
debugging = ["ipython"]
style = ["ruff", "black"]
testing = ["pytest", "pytest-cov"]
typechecking = ["mypy"]
all = ["chatbot[style,profiling,debugging,testing,typechecking]"]

Upon hatch run mypy, I get:

WARNING: chatbot 1.5.2b0 does not provide the extra 'debugging'
WARNING: chatbot 1.5.2b0 does not provide the extra 'profiling'
WARNING: chatbot 1.5.2b0 does not provide the extra 'style'
WARNING: chatbot 1.5.2b0 does not provide the extra 'testing'
WARNING: chatbot 1.5.2b0 does not provide the extra 'typechecking'
/bin/sh: mypy: command not found

[ofek]

This doesn't work for me when my project name overlaps with other project available on PyPI

Mention that here pypa/pip#11296

[ofek]

Thanks!